1. Field of the Invention
The present invention relates to a skew-feeding correcting apparatus which corrects skew feeding of a sheet and an image forming apparatus including the skew-feeding correcting apparatus.
2. Description of the Related Art
Typically, a skew-feeding correcting apparatus to correct skew feeding of a conveyed sheet to an image forming portion has been arranged for image formation without sheet-skewing by an image forming apparatus which forms an image on a sheet. In general, such a skew-feeding correcting apparatus corrects skew feeding of a sheet by correcting skew feeding at a top end of a sheet as abutting the top end of the sheet to a registration roller or a shutter and by conveying the sheet as nipping the top end with conveying rollers while keeping the state thereof. U.S. Publication No. 2008/0240821 discloses a structure in which an abutment member having abutment faces separated plurally in a width direction is arranged at the downstream side of nipping rollers. With this structure, when a sheet is skew-fed, skew feeding of the sheet is corrected as one end of a top end of the sheet being abutted to any of the abutment faces of the abutment member after the top end of the sheet passes between the separated nipping rollers and as the other end being abutted to another abutment face with swinging about the one end. Then, the sheet of which skew feeding is corrected is conveyed to the downstream side as the top end of the sheet being nipped by the nipping rollers. Further, Japanese Patent Laid-Open No. 2010-83649 discloses a structure in which a plurality of pairs of registration rollers having a first registration roller and a second registration roller respectively is arranged in an axial direction. With this structure, when a sheet is skew-fed, skew feeding of the sheet is corrected as one end of a top end of the sheet being abutted to a nip of any of the pairs of registration rollers and as the other end being abutted to a nip of another pair of registration rollers with swinging about the one end. Then, the sheet of which skew feeding is corrected is conveyed to the downstream side with rotation of the pair of registration rollers.
However, with the above-mentioned skew-feeding correcting apparatuses, there may be a case that a skew-feeding correction error occurs with corner creasing caused by buckling at a corner part of the sheet when the top end of the sheet to be conveyed is abutted to the abutment face or the nip of the pair of registration rollers from the corner part of the sheet.
In particular, recently, in a case that sheets are conveyed at high speed to increase productivity of an image forming apparatus (i.e., the number of image-formed sheets per unit time), sheet top ends are more likely to be damaged owing to large impact at the time of abutting of the sheet top ends to the abutment face or the nip of the pair of registration rollers. Further, recently, a variety of types of sheets (e.g., sheets having various basis weights and sizes and sheets having surface treatment performed like coating) have been used. Sheets having low stiffness (e.g., thin paper of which basis weight is 50 g/mm2 or less) is more likely to be damaged when sheet top ends are abutted to the abutment face or the pair of registration rollers.